Storage and pressure control of refrigerated liquefied gases



Sept. '29, 1964 D 3,150,495

E. E. REE STORAGE AND PRESSURE CONTROL OF 'REFRIGERATED LIQUEFIED GASESFiled Aug. 9, 1962 INVENTOR. E.E. R EED A T TORNEYS United States Patent3,150,495 STORAGE AND ERESSURE CGNTROL OF REFRIGERATED LIQUEFIED GASESEdwin E. Reed, Bartlesville, Okla, assignor to Phillips PetroleumCompany, a corporation of Delaware Filed Aug. 9, 1962, Ser. No. 215,9135 Claims. (Cl. 6254) This invention relates to storage of liquefiedgases and pressure maintenance thereof. In one aspect it relates tostorage of such liquefied gases as ammonia and liquefied petroleum gasesat atmospheric or substantially at atmospheric pressure.

The term liquefied petroleum gases includes such material as liquefiedpropane, liquefied butane, mixtures of these two hydrocarbons, andliquefied propane and liquefied butane or mixtures thereof with minoramounts of such other materials as ethane, ethylene, propylene,butylene, isobutane, isobutylene, pentane and amylene. Included withthese minor amount constituents are such gases as nitrogen, carbondioxide, and even traces of oxygen.

The materials ammonia, liquid ammonia, or liquefied ammonia as referredto throughout this specification and claims embody a major proportion ofammonia, such as 98 to 99 percent or more by gaseous volume, and suchminor amounts of one or more of hydrogen, nitrogen, argon, methane, orhelium when combined so as to make up the diiference, that is, 2 to 1percent or less by gaseous volume.

In the production of liquid ammonia and liquefied petroleum gases,retention of the liquefied material in closed vessels for storage and/or transportation presents many problems. Since these materials arenormally gases, that is, they are gases at ordinary atmospherictemperature and pressure, retention at atmospheric temperature andpressure requires uneconornically large vessels. For this reason suchmaterials are stored as liquids in containers at subatmospherictemperatures or at superatmospheric pressures, or both.

For storing these materials at subatmospheric temperatures provisionmust be made for cooling the materials and for maintaining them at thedesired temperatures.

For storage at superatmospheric pressures, it is merely necessary toprovide vessels having suitable wall thickness coupled with suitablefabrication. However, pressure vessels are expensive. Likewise,maintaining materials at subatmospheric temperatures is also expensive.In many instances, a compromise is made as regards temperature andpressure for storage of these materials.

According to this invent-ion, I propose to store such material asammonia and liquefied petroleum gases in vessels at atmospheric orsubstantially atmospheric pressure and at subatmospheric temperature. Ihave a method and means for storing these materials in the leastexpensive vessels, that is, at atmospheric or substantially atmosphericpressure, with expenditure of only a minimum for utilities to maintainsuitable subatmospheric temperature.

One important consideration in pressure and temperature maintenance inthe storage of these materials is the presence of small amounts of gaseshaving higher vapor pressures than the vapor pressure of the materialbeing stored. Such gases are sometimes known as uncondensable gases,uncondensables, etc. One method employed involves withdrawal ofuncondensable rich gas from above the surface of the stored liquidcontaining such gases in solution, compressing the withdrawn gases,cooling for condensation, and merely returning the condensate anduncondensed gas to the storage vessels. Another method employed ismerely to vent the undesired gases. This latter method obviously is tobe avoided since valuable gases are lost as well as the uncondensablegas or gases.

I have discovered an improvement over the just stated method ofoperation whereby I am able to maintain liquid ammonia or liquefiedpetroleum gas at suitable subatmospheric temperature for storage underatmospheric or substantially atmospheric pressure at less cost thanconventional. In this storage operation I withdraw vapor from thestorage vessel, compress the vapor and cool to produce condensate.During this condensation the condensate dissolves at least some of thedifficultly condensable normally gaseous impurities. I carry out a phaseseparation between condensate and uncondensed, com pressed gases. Ireduce the pressure of the condensate and gas phase separately inrefrigeration steps. Some vaporization with simultaneous chilling occursin the pressure reduction of the condensate. Following the pressurereductions of the condensate and of the uncondensed compressed gases 1admix the chilled expanded gases with the pressure reduced, partlyvaporized and chilled condensate. This admixing operation furtherdissolves a portion of the difiicultly condensable material in thepressure reduced and chilled liquid. Some pressure reduced gases stillremain as a gas phase and this gas and all liquid are introduced intothe liquid in the storage vessel at least near the bottom thereof sothat the introduced gas is permitted to bubble upwardly through the tankliquid to dissolve a further portion of the gas. In this manner amaximum amount of the difi'icultly condensable, high vapor pressurematerial is returned as liquid, i.e., in solution in the liquid contentsof the vessel, thereby minimizing the temperature reduction required forstorage at atmospheric pressure.

An object of this invention is to provide a method and apparatus forstorage of liquefied normally gaseous materials containing minor amountsof diflicultly condensable, normally gaseous materials at the minimumover-all cost. Another object of this invention is to provide a methodand an apparatus for storing such liquefied gaseous material containingminor amounts of diificultly condensable normally gaseous material at arelatively high temperature and yet sufficiently low for storage atatmospheric or at substantially atmospheric pressure. Still otherobjects and advantages of this invention will be realized upon readingthe following description which, taken with the attached drawing, formsa part of this specification.

The drawing illustrates, in diagrammatic form, an arrangement ofapparatus parts for carrying out the process of this invention.

In the drawinng, reference numeral 11 identifies a tank which can be astorage tank in a tank farm, a cargo tank on a barge or in anocean-going vessel. Reference numeral 13 identifies a dome or an uppergas containing portion of vessel 11. A conduit 15 communicates with dome13 for passage of gas to a compressor 17 powered by a prime mover orengine 19. The compressor outlet connects through a conduit 21 to acondenser 23 and the condensate therefrom passes on through a conduit 29into a phase separation vessel 31. Coolant or refrigerant enterscondenser 23 by way of a conduit 25 and leaves by way of conduit 27.Condensate leaves phase separating tank 31 by Way of a conduit 33 andpasses through expansion motor valve 37 and on through a conduit 35 andultimately is returned to the lower portion of the liquid phase 51 intank 11. The flow of condensate through conduits 33 and 35 is regulatedby the motor valve 37. A float apparatus 39 senses liquid level inseparator tank 31 and a signal from the float apparatus is passed tocontroller 55 which emits a signal responsive to the signal from float39 to manipulate the motor valve 37. Upon rise of liquid level inseparator 31 the controller emits a signal to open somewhat further themotor valve 37, and vice versa. In this manner a reasonably constantliquid level is maintained in separator 31. The gas phase from separator31 is passed through a conduit 41 and a motor valve 43 and is introducedinto the liquid phase flowing through conduit 35. The flow of this gasphase is regulated by a pressure conntroller 47 which communicates byway of a pressure tap 45 with the gas-containing space of separator 31and with the motor of the motor valve 43. Thus, upon an increase ofpressure in separator 31, motor valve 43 opens to pass an increasedquantity of gas.

Upon passage of liquid phase to motor valve 37, a reduction of pressureto about atmospheric occurs accompa nied by some vaporization of thecondensate and proportionate cooling. Thus, the fluid flowing throughconduit 35 is at a lower pressure and at a lower temperature than thecondensate in separator vessel 31. Similarly, vapor flowing throughvalve 43 is reduced in pressure to about atmospheric and proportionatelychilled and this chilled vapor dissolves to a marked extent in thechilled liquid flowing through conduit 35. Valve 49 is positioned inconduit 35 at a point relatively close to vessel 11. This valve 49 isnormally fully open and is used as a manual valve in case it is desiredto close off conduit 35. The tank storage pressure is intended to be atleast approximately atmospheric pressure. The gas phase in the dome 13is identified by reference numeral 53.

In one instance the liquefied petroleum gas comprising about 6.2 percentby volume ethane (C hydrocarbon) and 93.8 percent propane (Chydrocarbon) is stored in vessel 11. Storage pressure is approximately14.7 p.s.i.a. (pounds per sq. in. absolute) at approximately 53 F. Thevapor in dome 13 in equilibrium with this liquid phase contains 27.8percent C hydrocarbon and 72.2 mole percent C hydrocarbon. Thesepercentage compositions of the LPG as described herein are given interms of mol percent. A compressor corresponding to compressor 17withdraws 875 cubic feet per minute of vapor from the upper portion oftank 11 and compresses this vapor to a pressure of about 60 p.s.i.a. atwhich pressure the temperature of the gas phase is increased from -53 F.to about +50 F. A 125 horsepower compressor is required for compressionto the mentioned 60 p.s.i.a. A conventional Freon refrigeration systemis used for providing a coolant for use in a condenser corresponding tocondenser 23. The compressed gas is thus chilled by Freon refrigerationand the condensate and uncondensed gases pass on into a separator vesselcorresponding to vessel 31. The liquid phase passing through expansionvalve 37 and the gas phase passing through a valve corresponding tovalve 43 were pressure reduced, resulting in temperatures ofapproximately 53 F. At these temperatures the remaining condensate, thegas produced by vaporization of a portion of the condensate and the gasfrom expansion valve 43 are intermingled and a considerable proportionof the gas from conduit 41 is dissolved in the condensate. The mixedcondensate and undissolved gas pass on through conduit 35 into thebottom portion of liquid 51 in tank 11 at about atmospheric pressure. Bybeing introduced into the liquid phase at the bottom thereof the gas hasan additional opportunity on bubbling upwardly through the liquid todissolve in the liquid thereby further reducing the uncondensable gaswhich will ultimately be recycled through the compressor and condensersystem. The liquid and gas issuing from the separator vessel 31 havepressures of approximately 55 p.s.i.a.

While the above example illustrates the operation of this invention bythe use of refrigerant Freon it is not essential that this particularrefrigerant be used. Conventional cooling water can, if desired, bepassed through condenser 23 by way of the conduit 25 and withdrawn fromthe condenser by way of the conduit 27. In this case, however, thecompressor corresponding to compressor 17 will be required to compressthe withdraw gas to a pressure of about 380 p.s.i.a. at which pressurethe compressed gas will have a temperature of about 190 F. This lattercompression requires the expenditure of approximately 615 horsepower tocompress the 875 cu. ft. per minute of gas to the latter mentionedpressure. Water will enter condenser 23 at a temperature of about F. andleaves the condenser at about F. The temperature of the condensate anduncondensed gas in separator 31 is about F. at a pressure of about 360p.s.i.a.

When using Freon as refrigerant in condenser 23 for refrigerating the875 cu. ft. of gas per minute after compression to the hereinbeforementioned 60 p.s.i.a., the Freon system requires about 336 horsepower.This 336 horsepower plus the horsepower required for compression of the875 cubic ft. of gas per minute to 60 p.s.i.a. gives a total powerrequirement of 461 horsepower. This power requirement thereforepossesses an advantage of 154 horsepower per hour in favor of the use ofthe Freon for coolant in condenser 23 over the use of water as acoolant.

While I have hereinabove described the operation of the apparatus of myinvention for cooling and maintaining at a subatmospheric temperatureand at atmospheric pressure, an LPG comprising C and C hydrocarbons themethod and apparatus can be used equally well for refrigeration ofliquid ammonia. Liquid ammonia produced along the Gulf Coast contains asinert and difficulty condensable gases hydrogen, nitrogen, methane andargon. Ammonia produced in this area contains from about 2 to about 5milliliters of these inert gases in each gram of liquid ammonia. In oneinstance liquid ammonia produced in that area was stored at a pressureof approximately 45 p.s.i.g. Ammonia stored at said 45 p.s.i.g. pressurecontained in the vapor phase above the liquid ammonia about 1.2 volumepercent of these inert gases. This proportion of inert gas correspondedto approximtely 15.78 milliliters of inert gas per gram of ammonia. Theliquid phase at this 45 p.s.ig. pressure contains approximately 0.15milliliters of inert gas per gram of liquid ammonia.

In the Texas panhandle area ammonia produced from local gas containsfrom about 1 milliliter to about 2 /2 milliliters of inert gases pergram of liquid ammonia. In this particular instance the liquid ammoniawas stored r in at 18 p.s.i.g. (pounds per sq. in. gauge) pressure. The

inert gas in the ammonia comprised hydrogen, nitrogen, methane, argonand helium.

Portions of apparatus disclosed herein are thermally insulated, as forexample, tank 11, conduit 29, separator vessel 31 and all conduitsleading from vessel 31 back to tank 11.

The apparatus herein described withdraws gas from ammonia storagecontaining along with ammonia one or more of the above-mentioned inertgases, compresses the withdrawn gases, and condenses a portion of thecompressed gases. The condensate is largely liquid ammonia. Upon phaseseparating the condensate and uncondensed gas, then pressure reducingthem separately for chilling, and admixing the pressure reduced gas withthe remainder of the condensate and its produced gas, a substantialportion of the inert gases are redissolved in the liquid. The finalstream of liquid and gas are introduced into the bottom of the liquidammonia in its storage tank.

By continuous withdrawal of the gas phase over liquefied petroleum gas,or the gas phase over liquid ammonia, compressing, condensing,separating condensate from gas, separately reducing pressure on eachwith concomitant chilling, then mixing phases and introducing the mixedphases to the bottom of the stored liquid for a period of time,substantially all of the inert gases or high vapor pressure gases arereturned to the liquid phase from which they came. Thus, storage underatmospheric pressure is easily achieved.

While certain embodiments of the invention have been described forillustrative purposes, the invention obviously is not limited thereto.

That which is claimed is:

1. A method of maintaining a liquefied gas comprising a major proportionof a readily condensable normally gaseous material and a minorproportion of a more difiicultly condensable normally gaseous materialunder regulated storage pressure in a closed storage vessel whichcomprises the steps of:

(1) withdrawing vapor of said materials from the vapor space of saidvessel;

(2) compressing withdrawn vapor of step (1);

(3) cooling the compressed vapor of step (2) so as to condense at leasta substantial proportion of the readily condensable material;

(4) passing the material from step (3) to an enclosed separation chamberto separate same into condensate comprising principally said condensablematerial and vapor comprising principally said more difficultlycondensable material;

(5) passing the condensate of step (4) thru an expansion zone tovaporize a substantial proportion of same and provide a cool streamthereof;

(6) passing the vapor of step (4) thru a separate expansion zone toprovide a cool stream thereof;

(7) passing the cool stream of step (6) directly from its expansion zoneinto the cool stream of step (5) to form a mixed cool stream ofcondensate and vapor and cause a portion of said more difiicultlycondensable material to go into solution; and

(8) passing the mixed cool stream of step (7) directly into the lowersection of the storage vessel so as to bubble the vapor fraction thereofupwardly thru stored liquid to eifect further solution thereof in saidstored liquid.

2. The method of claim 1 wherein the pressure in said storage vessel ismaintained at substantially atmospheric pressure and the expansion insteps (5) an (6) is to a pressure just above the pressure in saidvessel.

3. The method of claim 1 wherein said readily condensable material isprincipally propane and said more diificultly condensable material isprincipally ethane.

4. The method of claim 1 wherein said readily condensable material isprincipally ammonia and said more difficultly condensable materialcomprises at least one gas selected from the group consisting ofhydrogen, nitrogen, methane, argon, and helium.

5. A system comprising in combination:

(1) a vapor-tight storage vessel having a vapor space in its uppersection;

(2) a vapor-liquid separation vessel;

(3) a first conduit having means therein for compressing vapor from saidvessel connecting the vapor space of (1) with the separation vessel of(2);

(4) a second conduit having an expansion motor valve therein conectingthe lower liquid section of the separation vessel of (2) directly withthe lower section of the storage vessel of (1);

(5) a third conduit having an expansion motor valve therein connectingthe upper vapor section of the separation vessel of (2) directly withthe second conduit of (4) downstream of the expansion valve therein;

(6) a pressure controller sensitive to vapor pressure in the vessel of(2) and in actuating control of the motor valve of (5); and

(7) a liquid level controller sensitive to liquid level in the vessel of(2) and in actuating control of the motor valve of (4).

References Cited in the file of this patent UNITED STATES PATENTS1,371,427 Kerr Mar. 15, 1921 2,001,996 Whitman May 21, 1935 2,059,942Gibson Nov. 3, 1936 2,550,886 Thompson May 1, 1951 2,887,850 Adams May26, 1959

1. A METHOD OF MAINTAINING A LIQUEFIED GAS COMPRISING A MAJOR PROPORTIONOF READILY CONDENSABLE NORMALLY GASEOUS MATERIAL AND A MINOR PROPORTIONOF A MORE DIFFICULTY CONDENSABLE NORMALLY GASEOUS MATERIAL UNDERREGULATED STORAGE PRESSURE IN A CLOSED STORAGE VESSEL WHICH COMPRISESTHE STEPS OF: (1) WITHDRAWING VAPOR OF SAID MATERIALS FROM THE VAPORSPACE OF SAID VESSEL; (2) COMPRESSING WITHDRAWN VAPOR OF STEP (1); (3)COOLING THE COMPRESSED VAPOR OF STEP (2) SO AS TO CONDENSE AT LEAST ASUBSTANTIAL PROPORTION OF THE READILY CONDENSABLE MATERIAL; (4) PASSINGTHE MATERIAL FROM STEP (3) TO AN ENCLOSED SEPARATION CHAMBER TO SEPARATESAME INTO CONDENSATE COMPRISING PRINCIPALLY SAID CONDENSABLE MATERIALAND VAPOR COMPRISING PRINCIPALLY SAID MORE DIFFICULTLY CONDENSABLEMATERIAL; (5) PASSING THE CONDENSATE OF STEP (4) THRU AN EXPANSION ZONETO VAPORIZE A SUBSTANTIAL PROPORTION OF SAME AND PROVIDE A COOL STREAMTHEREOF; (6) PASSING THE VAPOR OF STEP (4) THRU A SEPARATE EXPANSIONZONE TO PROVIDE A COOL STREAM THEREOF; (7) PASSING THE COOL STREAM OFSTEP (6) DIRECTLY FROM ITS EXPANSION ZONE INTO THE COOL STREAM OF STEP(5) TO FORM A MIXED COOL STREAM OF CONDENSATE AND VAPOR AND CAUSE APORTION OF SAID MORE DIFFICULTY CONDENSABLE MATERIAL TO GO INTOSOLUTION; AND (8) PASSING THE MIXED COOL STREAM OF STEP (7) DIRECTLYINTO THE LOWER SECTION OF THE STRANGE VESSEL SO AS TO BUBBLE THE VAPORFRACTION THEREOF UPWARDLY THRU STORED LIQUID TO EFFECT FURTHER SOLUTIONTHEREOF IN SAID STORED LIQUID.